This invention relates to hydraulic cement mixes in which improved properties are obtained through reduction of the water content of the mix. More particularly, it relates to mixes comprising a superplasticizing chemical admixture with minimal slump loss and entrained air, and good hardening characteristics.
It is a basic rule of concrete technology that greater compressive strengths are obtained for a given cement content when reduced quantities of water are utilized in hydraulic cement mixes. In portland cement compositions, therefore, it is highly desirable to maintain a very low water content in order to achieve high strength in the final product. Water reduction per se, however, results in a loss of workability or fluidity of a concrete or mortar mix. It is necessary for the plastic portland cement concrete mix to have fluid properties in order to effect uniform distribution of the ingredients, pumpability to transmit the mix to the needed structural site, and flowability to permit the plastic mix to be easily consolidated in the desired form. Therefore, chemical admixtures are employed to achieve an improved concrete mix with reduced water content while maintaining adequate fluidity for ease of placement and consolidation. Chemical admixtures used to reduce water content are commonly referred to as superplasticizers or high-range water-reducing agents. Among the known materials comprising high-range water-reducing admixtures are salts of the condensation products of naphthalene sulfonate and formaldehyde, such as disclosed broadly in U.S. Pat. Nos. 2,141,569, 3,667,780 and 4,308,069. Melamine sulfonates condensed with formaldehyde are disclosed in U.S. Pat. No. 4,394,175. Salts of lignosulfonates are described in U.S. Pat. Nos. 3,767,436, 3,864,290 and 4,367,094. Often set retarders are used to control and maintain a desirable fluidity for a desired time interval. An example is noted in U.S. Pat. No. 4,290,817 issued to Villa et al on Sept. 22, 1981.
Although the use of high-range water-reducing agents in concrete mixes may be desirable in some instances, there are some undesirable properties which tend to diminish the intended function of these agents. The favorable water reduction and workability obtained in mixes which contain naphthalene sulfonate condensates or melamine sulfonate condensates often is soon lost in the early stages of mixing and placing. Mixes containing the admixtures are often designed with six to eight inch slumps. The slump test is a measurement of concrete workability and is described in ASTM C 143, Standard Test Method for Slump of Portland Cement Concrete. Initial slump of six to eight inches of concretes containing admixtures is often reduced to less than three inches within an hour after initial mixing. The loss in slump, and corresponding loss of workability, is both undesirable and unsatisfactory. When slump is lost, the addition of water has been used to restore workability. Addition of water is known as retempering which results in a concrete mix containing more water than desirable which causes a significant loss in compressive strength after hardening and a number of other problems or deficiencies. The addition of an agent which retards early hydration and extends the time of setting of the plastic mix is an alternative means of maintaining a desirable slump in mixes containing high-range water-reducing agents. However, the extended time of setting often makes use of such a combination of admixtures undesirable. Among the known water-reducing agents, the lignosulfonates also exhibit set retarding properties. Lignosulfonates, when used in an amount sufficient to furnish the desired water reduction in a mix, normally entrain more air than desired and retard the setting time of concrete far beyond the ranges for a high-range water-reducing admixture.
For many concrete applications it is desired to have a mix which is reduced in water content in order to obtain high compressive strength. It is also particularly desirable in many applications to have a concrete mix which will harden quickly so that construction work can continue as soon as possible in the immediate area in which the concrete mix was placed. It is further desirable to have a concrete mix which will remain workable for a reasonable time, eg. roughly one hour, thereby avoiding the need to waste material not promptly used or sacrifice quality in terms of compressive strength by retempering. Formulating a mix of reduced water content results, as indicated, in a loss of the desired extended workability. Compensating for this loss by means of other admixtures has not been completely satisfactory in meeting the total objective. The addition of set retarders, as also indicated, will extend workability but prolongs the rate of hardening by a time greater than the time over which the mix is workable, thus resulting in a net time loss in terms of ability to continue work at immediate area.
Lignosulfonates used by themselves as water-reducing agents are completely unsatisfactory for the contemplated purposes since the set-retarding action of these material delays the rate of hardening well beyond the time benefits obtained in terms of increased workability.
One approach in our work to provide a new and improved admixture for the contemplated purposes was to combine commonly used lignosulfonates with various other water-reducing agents, but in these investigations it was still found that the rate of hardening was unsatisfactorily delayed, and in many cases the workability was also either unsatisfactory or marginal.
An object of the present invention is to provide water-reduced hydraulic cement compositions, particularly concrete compositions, enabling more efficient time use in construction.
Another object of the invention is to provide water-reduced cement compositions of time extended workability without delaying the rate of hardening for a time period greater than the time during which the mix is workable.
A further object is to provide water-reduced cement composition which will remain workable for at least about one hour while the rate of hardening thereof is prolonged by no more than one hour or even less.
A still further object is to provide cement compositions consistent with the above objects and exhibits other desired properties such as desired air contents for forming quality, high compressive strengths cement structures.